In many industrial environments, it is necessary to detect the level of product in a holding tank or bin. Level sensors are typically attached to the holding tank or bin, and electrically connected to remote gauges at a control room or other central location, where technicians or control systems may monitor the status of the bins to provide the appropriate process control.
Various technologies have been developed for level sensing. These include various contact sensing technologies using floats or drop weights, as well as various non-contact technologies such as reflecting electromagnetic radiation or ultrasonic vibrations from the surface of the product in the bin to determine the height of the product.
In some applications, it is particularly important to move the sensor away from the product. For example, in a foundry where the level of a hot melt of steel or ore is to be level sensed, it is particularly important to keep the level sensor a safe distance from the hot melt. In these applications, nuclear level sensing gauges are used.
In a nuclear level sensing gauge, a source of nuclear radiation is positioned on one side of the bin to be level sensed. A nuclear radiation detector is placed on the opposite side of the bin. The radiation exiting the source is in the shape of a wide generally vertically dispersed beam, directed toward the interior of the bin. The product in the bin substantially absorbs the radiation that impinges upon it. If, however, the bin is not full of product, some part of the beam of radiation from the source passes through the bin and exits from the bin on the side opposite to the radiation source, and irradiates the radiation detector. Because the product in the bin substantially absorbs the radiation that impinges upon it, thus reducing the amount of the radiation beam passing through the bin, the amount of radiation stimulating the radiation detector, is inversely proportional to the amount of product to the bin. Thus, the amount of radiation detected by the radiation detector, is compared to minimum and maximum values to produce a measurement of the amount of product in the bin.
FIG. 1 illustrates a typical prior art nuclear level sensing gauge, in which the nuclear detector is based on a scintillating crystal. An elongated scintillating crystal 14 produces photons of light when exposed to nuclear radiation from source S. The number of photons produced is related to the amount of radiation impinging on the crystal. To detect radiation passing through the bin, the scintillating crystal 14 is placed on the side of the bin opposite to the radiation source, with the long dimension of the crystal generally vertically oriented. A photomultiplier tube 12, used as a light detector, is coupled to an end of the crystal, and detects photons of light emanating from the scintillating crystal, and produces from this a signal for amplification by electronics 10, which produces an output indicative of the amount of radiation impinging on the crystal, and thus the level of product in the bin. This type of sensor is discussed in U.S. Pat. Nos. 3,884,288, 4,481,595, 4,651,800, 4,735,253, 4,739,819 and 5,564,487.
An improvement to the traditional nuclear gauge is disclosed in U.S. Pat. No. 6,198,103, filed by the assignee of this application. The '103 application discloses a nuclear level sensing gauge, seen in FIGS. 2A and 2B, which uses a bundle of one or more scintillating fibers as the radiation detector, in place of a scintillating crystal. In the version of FIG. 2A, the fibers are directly coupled to a photomultiplier tube 12, whereas in the version of FIG. 2B, the fibers are coupled to the PMT 12 via a light guide 18, which permits the PMT and amplifying electronics 10 to be positioned remotely from the fiber bundle 16.
The use of a scintillating fiber yields substantial improvements in cost, performance and ease of use and size and sensitivity configuration as compared to known gauges which use a scintillating crystal. Specifically, compared to a scintillating crystal, the scintillating fibers are light, can be easily coiled for shipment, and are easy to cut to desired lengths. Scintillating fibers can be readily curved to match the curvature of a particular bin, whereas crystals are rigid and difficult to custom manufacture. Also, scintillating fibers have better internal reflection characteristics than crystals, meaning that fiber scintillating sensors can be made longer with less loss than crystal scintillating sensors. Finally, a bundle of one or more fibers can have substantially less heat capacity than the corresponding crystal, meaning the bundle is more readily cooled.
Unfortunately, both the crystals and fibers exhibit light intensity losses when manufactured in long lengths. FIG. 3 illustrates the decay of light intensity as a function of the distance of a travel from a scintillation source through a medium, and the definition of the “attenuation length” L(i/e) of a medium, which is defined as distance that light that light can be transmitted through a medium before the light intensity is reduced to 1/e of its intensity at its origin. A fiber bundle typically has an “attenuation length” of about 2.5 meters. As can be seen from the FIG. 3 curve of light intensity vs. distance of travel, light loss is relatively severe at distances longer than the attenuation length, and nonlinear. However, fiber bundles and crystals have been used commercially at long lengths, up to 10 feet for crystals and 12 feet or longer for bundles. Crystals are practically limited to approximately 10 foot lengths because of the difficulty of manufacturing bars in longer sizes. Fibers are not practically limited by manufacturing constraints, but are constrained by the attenuation length of the polystyrene medium used to make the fibers.
Engineers confronting the limited lengths of scintillating crystals have created serialized devices that use multiple crystals for sensing level. FIG. 4 shows a typical prior art arrangement of this kind, in which a plurality of scintillating crystals 14 are placed in serial fashion adjacent a bin opposite to the source S, each crystal stimulating a photomultiplier tube 12 which is coupled to electronic amplifiers 10. The output of the various amplifiers are then coupled to summation electronics 20. Each crystal has a length less than the attenuation length of the crystal, but the serially positioned crystals have a collective length Lt that can be substantially greater than the attenuation length,
FIG. 5 shows an alternative serialized arrangement of crystals 14 that has been used in installations where it is desired to move the photomultiplier tubes 12 remote from the crystals 14; in this embodiment a light guide 18 couples light from each crystal 14 to each PMT 12. As in FIG. 4, the crystals are generally cut to a length less than the attenuation length, but have a collectively length Lt that can be substantially longer.
Unfortunately, the solutions illustrated in FIGS. 4 and 5 suffer from high complexity and cost, due to the replication of the PMT 12 and electronics 10 and the requirement for a summation electronics unit 20, rendering this form of gauge uncompetitive with a single fiber bundle in many environments; however, as noted, a fiber bundle suffers from attenuation losses at long lengths.
Accordingly, there is a need for an improved scintillating nuclear level sensing gauge which address the shortcomings of the existing products.